I think to prevent abusing it, it could automatically draw out all of the ki of the people giving to it, and immobilize them too, since they'd be concentrating on sharing their power. That way, to prevent it, all someone has to do is just come up to them and smack them with a melee hit, or blast them with a beam. Without ki and the ability to move, they're sitting ducks. Easy enough to stop, but if it does manage to get off, then it'll be powerful enough to be worth it.